In the previous post we looked at some OAuth security considerations. In this post we review the IANA considerations from RFC.
First, the token types have to be registered with the OAuth external review mailing list.
Examples could be included with the registration request. Registry updates must only be done by designated experts.
Registration templates include type name, authentication scheme, change controller and specification document.
Initial registry contents include

• client_id, 
• client_secret, 
• response_type, 
• redirect_uri, 
• scope, 
• state,
• code,
• error_description,
• error_uri,
• grant_type,
• access_token,
• token_type,
• expires_in,
• username,
• password
• refresh_token

Response types are also registered. Again these include response type name, change controller and specification document

OAuth testing should cover these registrations. For example, tests should cover the different response types and token types.

OAuth testing requires clients and users mapping to be tested. In addition, resource management policies should be tested. For example, resource management testing should include test that the public clients should don't have access to the user profiles. Similarly user profiles based access should have access to the non-user-profile specific information.

A specially designated admin client is useful for offloading the troubleshooting to non-development. teams. however, that may be for internal use and should ideally be built without modifying the existing protocol implementation.

OAuth security considerations from RFC continued.
1) resource owner password credentials - this is probably the one that grants maximum access to the client. However client and authorization server should limit the scope. This method in general is different from the pattern that this RFC proposes and is used for backward compatibility. The vulnerability here is that the resource owner does not have control over how the credentials are used.
2) tokens, codes, passwords, and secrets should not be transmitted in the clear. Http headers, URI appear as clear text. State and scope could appear in the clear so they should not ave any sensitive  information.
3) ensure endpoint authenticity by requesting TLS with server authentication. TLS certificates must be validated
4) tokens, passwords etc should not be guessable. Their probability should be fewer than one in 2^128
5) due to the use of redirects there could be phishing attacks possible. Websites that ask for credentials should be authenticated.
6) Cross site request forgery should be prevented such as when  a  user-agent is made to follow a malicious URI to a trusting server.  In such security attacks, the attacker injects his own authorization code. CSRF protection is achieved by including a value in the URI that hints at the authenticated state.

URL shortening for REST API

REST APIs are easy to document. And their use often involves small variations in the URI. Therefore , URI shortening and parameter abbreviation helps particularly on mobile devices.  The difference between a regular URL shortening and this kind of smaller URI's is that these will not necessarily be a hash. The URI's can have meaningful abbreviations given that we could  control which API's they map to  and because the APIs are limited. The shortened URI' s could  be stored in a database and the URI's they map to could  still be available independently. This means that we can develop the MVC route based API's as usual and have this service as an add on. The service merely redirects to the actual API"s as HTTP 302 and not require any static or dynamic pages assigned to the short URIs. This service could even live in the http proxy, if the API uses one. The proxy can be transparent on this redirection.
API documentation has to be kept in sync with this short URI mapping. Documentation can be generated from the config routes in which case the short APIs may need their own listing and correspond API documentation redirects. if the framework were to allow such short URI services, then thereis no need for an external service or database and short URi's  could be provisioned in the config files itself. This will also help keep the documentation together.
Query string or Request body parameter abbreviation requires changes on the API implementation. Parameters need not be specified with their full names but by their first initial or some shortener convention.  Only the parameter lookup changes so they can be kept in sync with their corresponding actual parameters. Together with abbreviations, short URI and documentation provide compelling convenience. Statistics could be gathered for both short and regular URI's to warrant continued usage.

This is  a quick overview of the OAuth RFC 6749 and will cover some salient features mentioned there.
Implicit workflow is described as follows:
The client is issued an access token directly instead of the authorization code. This is done on the basis of resource owner authorization The client is issued an access token. Client is not verified.  The access token is sent back in the uri fragment. Unauthorized parties could gain access to it. In some cases client can be verified with the redirect uri. Access tokens are issued with the minimal scope.  Phishing attacks are possible where the access token can be switched with the another token previously issued to the attacker. Due to such security considerations, when authorization code grant is available, implicit might not be provisioned.
Other security scenarios include the following:
1) A malicious client can impersonate another client and obtain access to protected resources. This can be avoided with the registration of redirect uri and receiving authorization responses. The authentication server must authenticate the client whenever possible.
2) Access tokens must be kept confidential in transit and storage. TLS can be used for transit and database could be encrypted.
3) Authorization servers may issue refresh tokens to both web application and native application clients. Refresh tokens must be issued only to the client who was previously issued the token. The authorization server could also use refresh token rotation where the clients who used the previous token can be identifed.
4) Authorization codes must be short lived since it is used for the exchange of a token.
5) The redirect_uri parameter can be manipulated.

In the previous post, there was a mention for cross user access or admin access, however in this post we talk about infrastructure support for mobile devices. One of the things we discussed was expiration of access. With mobile devices and other applications, there can be convenience provided to the user such that her login efforts are minimized. For example the authorization website where the user login to grant access to a client, may choose to remain signed in for the duration of a session that is longer the token expiry time.  Clients in expiry of their tokens need not request the user to login again. That can be maintained by the site. Further the user may grant indefinite time access or until explicit revoke at the website such that the client could continue to have the web site send redirects. Strictly speaking this is not OAuth. It's just a convenience provisioned outside the OAuth.

OAuth testing discussion continued.

In the previous posts we discussed that OAuth testing depends on the nature of the token. The token is granted to a client for access to a user's resources. Therefore, OAuth APIs are largely qualified by user first and then client. So APIs are resource qualified in the form '/v1/users/me/clients/clientId'.  The reverse order of '/v1/clients' is generally not needed to be maintained because the clients don't have any access unless a user authorizes a resource.
That said, it is important to consider admin access such as one that can reach across users. Is it necessary to have admin access ? or should users manage their own client authorizations ? This is the topic we will briefly consider.
Resources that are not user-specific are generally less demanding on security as opposed to resources that are user owned. For example, the reward cards and balances on them that a user has is considered confidential and so no other user should have access to the same. However, what if there is governance required such as when the law enforcement agencies want to know how may coffee drinks you need to have before you get your free one ? Or even more pertinent where was your last purchase store in case you died ? Governance has a notorious connotation to big brother snooping but in our technical discussion here we refrain from such discussion and consider the case where it is important for another user or system to know information of other users. This could be as benign as friends sharing card information to see which card needs to be used first for the free drink. In OAuth spec, there is no limitation against admin access or access to other users information. In OAuth there is no mention for grouping users or clients. In OAuth there is no mention for API aliasing or shortening or parameter abbreviation for ease of use by one or more users especially from handheld devices. This is left to individual vendors to determine.
In the case where vendors choose to have cross user access or admin client access, the nature of the token granted is relevant. This could be scoped to include certain information such as last store while excluding certain information as credit cards or social security number. Basically, the protocol does not and should not prevent granularity of access and accessors be it the client or the user.
This definitely calls for OAuth tests so that security, functionality and performance are not affected. The RFC discusses several security implications from the user and client point of view however testing may need to be customized to the implementation. The implementation can be as simple as requiring all authorizations owned and delegated to the end user. 

Bearer token recommendations from RFC
1) Bearer tokens must be safeguarded since it gives access to the bearer. It should not appear in the clear such as in the header or in cookies. It should be passed only over secured traffic.
2) TLS certificate chains should be validated otherwise DNS hijackers can steal the token and gain unintended access
3) https should be used for all OAuth communications. The transport layer security is necessary for encrypting the traffic and securing the endpoints.
4) Bearer tokens should not be stored in the cookies. Implementations must not store bearer tokens in cookies because it can lead to cross site forgery attacks.
5) Bearer tokens should only be issued as short lived. One hour or less is recommended. Using short lived bearer tokens means that very few will gain access to it to misuse it.
6) Bearer tokens should always be scoped, scoping their use to the designated user or party. This is important because we don't want to grant universal access.
7) Bearer tokens should not be passed in page URLs since browses, web servers, and other software may not adequately secure URLS. The token may appear in web server logs and other data structures.
Appropriate error codes must be returned to deny specific requests. These include:
1) invalid_request - resource access error response This covers cases where the request is to be denied on grounds of invalid user or client. Http Status code is 400
2) invald_token - This is also a resource access error response. This covers the case where the token may have expired or the client has used a fabricated token. Http Status code is 401 (unauthorized)
3) insufficient_scope - this is also a resource access error response. This covers the case where the token has not been scoped and may cause security vulnerability. Http Status code is 403 (forbidden).
An example of a successful response could be
     HTTP/1.1 200 OK
     Content-Type: application/json;charset=UTF-8
     Cache-Control: no-store
     Pragma: no-cache

     {
       "access_token":"mF_9.B5f-4.1JqM",
       "token_type":"Bearer",
       "expires_in":3600,
       "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA"
     }